Physiological, anatomical and clinical studies suggest two major ascending pain systems originating from fibers in the mammalian ventrolateral spinal cord: (1) a direct spinothalamic system projecting to the posterior (PO) and intralaminar thalamus and (2) a spino-recticulo-thalamic system projecting via the medullary nucleus gigantocellularis (NGC) to the centre median-parafascicular (CM-Pf) thalamic complex. If each pathway is necessary for normal pain sensation, lesions selectively interfering with transmission along either of these pathways should reduce behavioral responses indicative of pain. Accordingly, quantitatively controlled noxious and innocuous thermal stimuli will be used in an operant escape paradigm to test the long-term effect of lesions within the NGC, PO and CM-Pf of cats and squirrel monkeys. Threshold-independent increases in escape latency will be used to detect motor deficits precluding normal responses. The functional selectivity and long-term electrophysiological effects of such lesions will be examined by periodic evoked potential recordings from electrodes chronically implanted within PO and CM-Pf. Comparison of the behavioral and electrophysiological effects of the lesions should permit identification of the ascending system(s) necessary and/or sufficient for pain.